Method of making article from metallic powders

ABSTRACT

A method of making a continuous strip at rapid rates from metallic powders, which powders are devoid of non-metallic binders. This is accomplished by the simultaneous compacting and resistance-heating welding of the powders being fed between electrically energized rolls. The roll load coupled with a low voltage - high amperage current therebetween assures a highly dense metallic strip suitable for further metallurgical processing. Means are also provided to assure a full width of uniformly dense metallic strip substantially equal to the width of the fed metallic powders.

o Muted States Patent 1 1 [111 fifiamse LaTour 5] May 1, 1973 METHOD OFMAKING ARTICLE 3,359,100 12/1967 Claus et a1. ..75 200 x FROM METALLICPOWDERS 3,290,145 12/1966 Daugherty ..75/200 X 3,250,892 5 1966 I ..219149 [75] Inventor: Harry LaTWr Middletown Ohio 3 194 858 7/1965 s ii'fhheim .15 260 x [73] Assignee: Armco Steel Corporation,

Middl t Ohi I Primary Examiner-J. V. Truhe Assistant ExaminerL. A.Schutzman [22] Flled' June 1971 Attorney-John W. Melville et a1. [21]Appl. No.: 156,651

[57] ABSTRACT [52] US. Cl. ..219/149, 75/200, 75/214, A method of makinga continuous strip at rapid rates 75/226, 29/1822, 219/76 from metallicpowders, which powders are devoid of [51] Int. Cl. ..B21j 1/06non-metallic binders. This is accomplished by the [58] Field of Search..2l9/l49, 148, 91, imultaneous compacting and resistance-heating weld-9/ 6, ing of the powders being fed between electrically energized rolls.The roll load coupled with a low voltage high amperage currenttherebetween assures a 1 References Clted highly dense metallic stripsuitable for further metallurgical processing. Means are also providedto assure UNITED STATES PATENTS a full width of uniformly dense metallicstrip substan- 3,567,903 3/1971 Parker ..75/226 X tially equal to thewidth of the fed metallic powders. 2,906,596 9/1959 Ballhauser.........75/226 X 3,340,052 9/1967 lnoue ..75/200 19 Claims, 11 DrawingFigures Patented May 1, 1973 5 Sheets-Sheet l INVENTOR/S AMRRY ATTORNEYS5 Sheets Sheet 2 INVENTOR/S Mill? Wm ATTORNEYS Patgnted- May 1 1973 Y 73,731,050

5 Sheets-Sheet 4 Fin: :1

INVENTOR/S BY M VM, 2;; nu/25 ATTORNEYS Patented Ma 1, 1973 5Sheets-Sheet 5 NVENTOR/S x/mfy Aral/e ATTORNEYS METHOD OF MAKING ARTICLEFROM METALLIC POWDERS BACKGROUND OF THE INVENTION This invention relatesto a method for the rapid production of a continuous strip of metal frommetallic powders. I-Ieretofor the subject of powder metallurgy was usedin conjunction with the production of single items, or at most a slowspeed continuous process. Further, said processes were of the type knownas oven-sintering or induction sintering.

Briefly, each of the aforementioned process are ones involving at leasttwo operations, namely, compaction followed by sintering. That is, thepowders are compacted at a first location and subsequently treated ortransferred to a second location in the green condition where theproduct is sintered. A third location is also utilized for further hotor cold working to increase density. Such procedures have been widelyused since it was readily observed that such products exhibited anexcellent finish with excellent tolerances. This represented a savingsover using a forged product which requires cutting and/or machining toachieve the final product. Also, by the elimination of the step ofhaving to remove metal, crack propagation was substantially reduced.

While these procedures represented some savings over products producedby other processes, there were factors in the operations which tended toadd to other costs. For example, the two procedures require expensivedies and equipment along with operating times up in the hours. Anotherdrawback to these procedures is the fact that it was often necessary toincorporate a binding agent along with the metallic powders in order togive the product some green strength sufficient to subsequently treat ortransfer the item to the sintering station. It was necessary then duringthe sintering operation to burn off the binding agent from the compactedpowders. As a consequence, it was often difficult to obtain the desireddensity in the sintered product.

Quite recently, a new procedure was developed to overcome several ofthese disadvantages and to produce a rather highly dense sinteredproduct. This procedure has been defined in the art as spark sintering"or spark discharge". Spark sintering, for example, has been defined as aprocess for making powder metallurgy parts by the simultaneousapplication of a moderate amount of force on said powder whilesubjecting the powder to a combination of DC and AC current. While theprecise theory is not known at this time, it is believed that while themetallic powders are under the limited load, the electric current whichis being passed therethrough results in a spark discharge betweenadjacent surfaces of metallic particles such that fusion betweenparticles occurs. Such a procedure is characterized by a combination oflow pressure and electrical characteristics of high voltage and lowamperage. Like the earlier procedures, the latter one is a relativelyslow operation making it unsuitable for a continuous operation. Foradditional information reference may be made to the five INOUE patents,namely, U.S. Pats. Nos. 3,250,892; 3,241,956; 3,317,705; 3,340,052 and3,387,972.

In contrast to the procedure just described, the present inventioncontemplates a process whereby continuous strips can be produced at arapid rate from metallic powders by means of a combination of relativelyhigh pressure and low voltage high amperage electricity. The resultingproduct approaches the theoretical density and is accomplished withoutthe use of binding agents admixed with the metallic powders.

SUMMARY OF THE INVENTION Briefly in the practice of this invention, ametallic powder, such as low carbon 18Cr-l 2Ni-2.5Mo stainless steel, ofa size passing through a 325 mesh screen, is subjected to a simultaneouscompaction and welding of the metallic powders between electricallyenergized rolls. The latter operation is characterized by theapplication of a heavy roll load and the electrical characteristics oflow voltage and high amperage.

By utilizing loads well in excess of those contemplated in similar priorart procedures, more intimate contact is achieved between the particlessuch that with the application of the electrical current it is possibleto rapidly heat the entire mass of the particles through resistanceheating up to the welding temperature needed to fuse the product betweenthe rolls.

The further advantages of this operation will become apparent in thedetailed description thereinafter.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic representation ofthe process of this invention, including the subsequent operations ofrolling and annealing.

FIG. 2 is a sectional view of a second embodiment of this invention,said embodiment utilizing a traveling mold to contain the metallicpowders during compaction.

FIG. 3 is an enlarged sectional view taken through the electrode rollsand traveling mold of FIG. 2.

FIG. 4 is a schematic representation of a third embodiment wherein aplurality of opposing rolls are arranged in tandem relationship.

FIG. 5 is a schematic representation of a pre-compaction system for usein the practice of this invention.

FIG. 6 is a simplified sectional view of an alternative manner ofelectrifying the compaction welding rolls which are segmented.

FIGS. 7a and 7b are simplified perspective views showing two metallicpowder feeding systems to vary the depth of metallic powder being fedinto the compaction welding rolls.

FIG. 8 is a sectional view showing the variation in depth of metallicpowder, which variation may be achieved by the systems shown in FIGS. 7aand 7b.

FIG. 9 is a front elevational view of opposing contoured compactingwelding rolls, the counter being exaggerated for illustration purposes,for use in the practice of this invention.

FIG. 10 is a perspective view, a portion of which is cutaway, of thecompacting welding rolls, such as in the system illustrated in FIG. 2,but incorporating powder dividers preceeding said rolls.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS This invention relates ingeneral to the production of a continuous-uninterrupted strip of metalproduced from metallic powders, devoid of non-metallic binders; moreparticularly, it relates to the production of said strip having auniform density throughout its width.

The foregoing is accomplished by a unique process involving thesimultaneous compacting and resistance heating to weld the powder intostrip form. It will be appreciated that the compacting pressures andelectrical characteristics will vary over a range of values depending onthe powder being processed. Nevertheless, since this is in effect acontinuous welding operation, wherein the heating is effected byresistance heating of the metallic particles, it was determined thatwith increasing compacting pressures the resistivity of the particlesdecreased thereby requiring a lower voltage across the welding zone.However, it is believed a person skilled in the art can readily selectthe parameters necessary to achieve the results herein in accordancewith this invention.

While the further description will be primarily directed to theproduction of a strip of low carbon l8Cr-l 2Ni-2.5Mo stainless steel orTi-6Al-4V titanium alloy, it should be understood that other metallicpowders may also be processed by the procedures to be described.Therefore, no limitation is intended to be imposed on this invention,except as set forth in the appended claims.

Referring now to FIG. 1 on the accompanying drawings, there is shown ahopper 10 adapted to continuously feed metallic powder verticallybetween the pair of electrode rolls 12, 12a ofa compacting mill. Thepressure exerted thereby is represented by the opposing arrows and PSupplied to said hopper 10 is a metallic powder free of non-metallicbinders.

The rolls 12, 12a are energized by the electrical circuitry shown so asto heat the powder between the nip of the rolls by resistance heating tothe welding temperature. The circuit may be AC. or D.C., and ischaracterized as low voltage high amperage.

Since this invention is dealing with the simultaneous compaction andwelding of metallic powders, devoid of non-metallic binders, it may bedesirable to include some edge restraining means on each side of thestrip. While a specific means is not contemplated as a limitation on theinvention, effective results have been achieved with side rails orguards, and rolls, the latter being free wheeling as opposed to driven.

While the type and thickness of the compacted powder strip will affectthe speed of the process, it is contemplated that rates over a broadrange may be effectively followed. Thus, the roll speed of rolls 12,12a, which are driven, shall vary.

As the metallic powders pass between the rolls, they are compacted andheated to the welding temperature to emerge as a highly dense strip 14.While this procedure is geared to produce a high density product, thereare practices which can be followed to improve or increase said density.For example, the use of powders of a relatively uniform size, smallerdiameter rolls in relation to the strip thickness, and a thinner stripare factors which can be used to increase density.

On materials where oxidation may be a problem at the weldingtemperation, gas purging nozzles 16, 16a may be provided. In otherwords, by the use of said nozzles, or other suitable means, it ispossible to control the environment about the compacting weldingoperation. Dry hydrogen, having a dew point no greater than 40C., issuitable for the continuous welding of stainless steel.

The hot emerging strip IQ is conveyed away from the compacting weldingoperation and may be directly coiled for use in the as welded condition,or processed further to effect additional metallurgical changes in thestrip. For instance, the strip may be passed through a cooling station18 to be quenched, or merely gas cooled. In any case, controlled coolingmay be effected in a variety of mediums. If greater densities, and/orsmoother finishes are desired on the strip, a cold rolling station 20may be employed in the system. Finally, said rolling may be followed bya further heating or annealing at a heat treating station 22 prior tothe coiling of the strip.

While the preceding are typical operations in the processing of metallicstrip, it should be understood that sequence changes, additions orvariations may be made without departing from the scope of theinvention.

FIG. 2 represents a second embodiment to the compacting weldingoperation of FIG. 1. In contrast to the system of FIG. 1, the rolls 30,300 are disposed in a vertical plane in spaced relationship. Between therolls a continuous traveling mold 32, or a mold of predetermined lengthif discrete strip lengths are required, is disposed to receive thepowdered metal from a hopper 34.

The mold 32 is U-shaped such that the opening 36 therein issubstantially equal to the width of the proposed strip 14a as seen inFIG. 3. To insure full width compaction of the powder within the mold,the upper roll 30 should be designed to substantially engage the opening36. Thus, by this embodiment, i.e., feeding the powdered metal into theopening 36 or trough of the mold 32, one can eliminate many of theproblems which may be encountered with the flow of powder into the nipof the rolls of the horizontal mill shown in FIG. 1.

FIG. 4 is a schematic representation of a modified procedure foreffecting the compacting and welding in stages. This system may be usedwith either the horizontal or vertical mills of the embodiments shown inFIGS. 1 and 2.

It will be observed that in this latest system a plurality of pairs ofrolls are disposed in tandem relationship. For convenience, three setsof rolls are illustrated and have been designated 40, 4011;42, 42a; and44, 44a. As explained previously, since the operation herein is welding,and it is by means of resistance heating of the particles, it wouldfollow that with increasing pressures or loads (P, P P;,), theresistivity of the particles to be welded decreases. That is, theparticles become more compact with fewer and smaller voids therebetweenand therefore a more direct contact between adjacent particles results.As a consequence, the voltage requirements decrease across rolls 40,40a; 42, 42a; and 44, 44a, respectively (V, V V

To help demonstrate the effectiveness of this invention, a strip of lowcarbon l8Cr-l2Ni-2.5 Mo stainless steel was produced by a system similarto that illustrated in FIG. 2.

EXAMPLE 1 Low carbon l8Cr-l2Ni-2.5Mo stainless powder (100 percent at325 screen size, 7.25 microns) was secured and the analysis was asfollows:

Cr 17.55% Ni l 1.35% C 0.030% Mn 0.10% Mo 2.20% Si 0.98% Fe balance.

emerge from between the rolls having a thickness of about 3.0mm, with awidth of about 7.3mm. The strip width was less than the top roll widthdue to the loss of pressure at the outside edges of the convex top roll.

In the as welded condition, the strip was tested for tensile properties.These results may be seen in the TABLE I along with a duplicate sampleannealed in dry hydrogen and slowly cooled.

TABLE I Strip Condition YS TS Hardness Elongof Sample MN/m MN/ RB Aswelded 209 Dry H, Anneal 305 459 81 7 Without any cold reduction ofeither sample, densities on the order of about 94 percent of the soliddensity were achieved. Increasing the roll load and/or the addition of acold rolling stage should substantially increase the density such as toapproach the theoretical density.

While some edge restraining means could have been used effectively toimprove the lateral dimension and character of the strip, other meansacting normal to the strip have been found to produce uniformly densestrips from edge to edge. However, said means will be discussedhereinafter.

Turning now to FIG. 5, there is shown a horizontal system wherein aprecompaction mechanism 50 is employed preceding the compacting weldingrolls 52,

52a. Such a system permits higher feed rates, hence, 55

faster mill speeds.

The precompaction mechanism 50 comprises a pair of opposing metallicbelts 54, 540, each of which incorporates one of said rolls 52, 52a. Thebelts 54, 54a, are

driven by rolls 56, 56a, respectively and are arranged in a commonvertical plane, but skewed as to their horizontal relationship. In otherwords, said belts are tapered at an angle, which may vary from severaldegrees up to about 8 to converge at the roll bite of rolls 52, 52a. Theupper roll 56 is displaced from roll 56a so as to permit the metallicpowder feed from hopper 57. Finally, disposed within each said belt,

there are provided a plurality of pairs of opposed pres sure rolls 58a53m acting to precompact the metallic powders prior to their entrybetween rolls 52, 52a.

It was noted earlier that some difficulty was encountered in achieving afully compacted strip from edge to edge. This has been overcome byregulating the temperature across the strip in the welding zone.

The first of several such means forming a part of this invention isillustrated in FIG. 6. Each of the compacting welding rolls 60, 60a arecomprised of segments 62a 62n, and 64a 6411, respectively, with opposingpairs of segments individually energized. In other words, each saidpair, i.e., roll segments 62a, 64a, constitute a set of compactingwelding rolls, which when employed with other pairs of like charactercan be used to cover the entire width of a proposed strip 66. Adjacentroll pairs are insulated from each other by insulation spacers 68a 68mand from the central shafts 70, 70a. This will insure that theindividual controls are effective.

The electrical powder leads 72a 72n may be secured to the individualroll segments in any well known manner, and energized by means of aprimary of a power transformer '74. Means such as a rheostat may beemployed in the several circuits to control the power therethrough.

It was found that in the typical single pair rolls, hot spots developedat the center between the said rolls. For example, a temperature profiletook on the appearance of an hour glass. But with the presentindividually controlled segmented rolls, it is possible to avoid saidhot spots by reducing the power to the middle segments. In other words,temperature uniformity can be obtained by regulating the power levelacross the width.

FIGS. 7a and 7b represent a different approach to ef fect densityuniformity in the welded and compacted strip from edge to edge. Forexample, it has been found that in resistance welding of metallic powderthere is an inverse relationship between resistance and density. Thatis, resistance decreases with an increase in density. Further, densityis directly related to the feed rate of the metallic powder.Accordingly, resistance decreases with an increase of the feed rate. Itis upon this latter principle that the systems disclosed in FIGS. 7a and7b operate to insure density uniformity throughout the strip width.

In FIG. 7a, a single feed hopper is positioned over the conveyor belt 82delivering the metallic powder to the compacting welding rolls. Disposedbeneath said hopper 80 are a plurality of strips 34, whose width may bebroad or narrow, such as plastic or Teflon, the latter being aregistered trademark, which may be moved in or out to expose a greateror lesser area of the underlying conveyor. This naturally results in avariable amount of powder being deposited onto the conveyor. FIG. 8shows a preferred profile of metallic powder as it is positioned priorto its entry into the roll bite of the compacting welding rolls. it isobvious that the powder depth profile can change markedly; however, bythe general configuration of FIG. 8 one can compensate for edge densitylosses, or increase the edge temperature.

FIG. 7b represents a variation to FIG. 7a but which produces the sameresult. Here, a plurality of feed belts 86a-86 with complimentaryhoppers 88a 88m are each arranged so as to deposit metallic powder ononly a limited portion of conveyor 90. By the simple procedure ofadjusting the feed and speed of belts 36a Son, it is possible to varythe depth of metallic power from edge to edge. For example, by operatingthe outermost feed belts at a higher speed than the innermost, aconfiguration can be developed similar to that shown in FIG. 8.

FIG. 9 not only depicts another means for achieving density uniformity,it represents a departure from normal rolling practice. Typically, arolling mill for reducing conventional strip material comprises a pairof rolls which are center crowned or possess a slightly convex surface.In contrast, to this, the compacting welding rolls 92, 92a of FIG. 9 areeach provided with a convex surface 94 or negative contour. That is, thediameter through the center is less than the diameter at each edge.Thus, by allowing the powder 96 to compress more on the edges than inthe center, the temperature and pressure will allow full densitycompaction up to the edge of the rolls 92, 92a. This may be demonstratedin the following example.

EXAMPLE II A titanium alloy (Ti-6Al-4V) with 6 percent aluminum and 4percent vanadium (l+400 mesh) was selected for use on contoured rollssuch as shown in FIG. 9. Using a 12.7mm width roll with 0.127mmconcavity (circular segment), a full 12.7mm width strip was produced. Onthe other hand, under substantially identical conditions, in using aflat contour on 12.7mm rolls, a strip width of only approximately 9.5mmresulted.

While the use of contoured rolls resulted in a 33 percent improvementover the flat rolls, it should be made clear that such increases can notbe attributed automatically to a given set of rolls irrespective of rollsize. Nevertheless, this demonstrates that improvements of a major orderare possible.

A final but different mechanism for achieving uniform density fromedge-to-edge is illustrated in FIG. 10. Such a system may be used withthe traveling mold concept described above with respect to FIG. 2. Itwill be recalled that the metallic powder is deposited on a mold 100 ata location preceding the compacting welding rolls 102, 102a, where thesaid compacting and welding takes place. However, it has been found thatwith such a mechanism, there is a tendency for the loose metallic powderto move in a circular pattern, i.e., follow the flux path between thecompacting welding rolls I02, 102a, in the traveling mold 100. Thepowder tends to move around the edge of roll 102 to the exit side. Thisloss of powder results in a width reduction.

To prevent said movement dividers or baffles 104 are provided to isolatethis effect and allow the full strip width to develop. While experiencewill dictate the number of dividers needed, it is contemplated that aplurality will be required to effect the isolation. Finally, thedividers 104 are curved at the roll bite end 106 to insure asatisfactory channeling of the metallic powders into the compacting andwelding zone. The curved end 106 is such as to compliment the curvatureof roll 102, but without interfering with the operation thereof.

While the invention has been described in relation to its most preferredembodiments, it should be recognized that numerous variations may befollowed in the system without departing from the spirit and scopethereof. Accordingly, no limitation is intended to be imposed on thisinvention except as set forth in the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows.

l. A method of producing a continuous metallic strip from powdered metalcomprising the steps of selecting a metallic power free of non-metallicbinding agents, feeding said powder between opposed compacting electroderolls, simultaneously applying pressure by means of said rolls tocompact said powder and a low voltage high amperage current thereacrossto resistance weld said powder into a highly dense continuous strip.

2. The method claimed in claim 1, wherein an open sided traveling moldis interposed between said rolls to receive said metallic powder.

3. The method claimed in claim 2, wherein said mold is U-shaped and oneof said rolls is adapted to ride within said mold.

4. The method claimed in claim 1, including the steps of cooling saidstrip and winding said welded strip into coil form.

5. The method claimed in claim 1, wherein said rolls are preceded by atleast one pair of opposed strip compacting electrode rolls whose voltageis greater than said first noted voltage, said rolls effecting apre-welding of said metallic strip.

6. The method claimed in claim 5, wherein there are a plurality of pairsof initial rolls, each pair of which is characterized by a voltagedecreasing in the direction of feed of said metallic strip.

7. The method claimed in claim 6, wherein one roll from each said pairis disposed within an encircling belt, and said belt engages thepre-welded metallic strip.

8. The method claimed in claim I, wherein said welding-compacting isconducted in a controlled environment substantially free of oxidepromoting conditions.

9. The method claimed in claim 8, wherein said environment issubstantially a dry hydrogen atmosphere.

It). The method claimed in claim 1, including the step of varying thegap between said rolls, whereby to insure a uniform and fully densestrip from edge to edge.

11. The method claimed in claim 10, wherein said variation is achievedby providing said electrode rolls with a uniform negative crown from oneend to the other.

12. The method claimed in claim ll, including the step of depositingsaid powder metal on a conveyor, whereby said conveyor is positioned tofeed the powdered metal directly between said compacting electroderolls.

13. The method claimed in claim 12, including the step of varying thedepth of said powdered metal on said conveyor from one edge thereof tothe opposite edge.

14. The method claimed in claim 13, wherein said variation is achievedby means of a plurality of feeder hoppers, each said hopper depositingsaid powdered metal at a predetermined rate on a preselected portion ofsaid conveyor.

15. The method claimed in claim 13, wherein said variation is achievedby interposing flat strips between the feeding source and said conveyor.

16. The method claimed in claim 12, including the step of subjectingsaid powdered metal to a compacting operation preceding saidcompacting-welding.

17. The method claimed in claim 1, wherein said welding is accomplishedin a plurality of zones transverse to said strip, and that each saidzone is individually controlled so as to vary the current passingthrough said strip from one edge to the opposite edge.

18. The method claimed in claim 2, including the steps of isolating saidpowdered metal into a plurality of narrow channels from a locationpreceding up to the said compacting-welding.

19. The method of producing a metallic article from powdered metalcomprising the steps of selecting a metallic powder free of non-metallicbinding agents, placing said powder between opposed compactingelectrodes, each said electrode characterized by a plurality ofindividual zones, each zone connected in electric circuit with one zoneof the opposing electrode, simultaneously applying pressure by means ofsaid electrodes to compact said powder and a low voltage high amperagecurrent thereacross to resistance weld said powder into a highly densearticle, with said current selectively controlled in each pair ofelectrically connected zones so as to vary the current flow betweenadjacent zones in a given electrode.

1. A method of producing a continuous metallic strip from powdered metalcomprising the steps of selecting a metallic power free of non-metallicbinding agents, feeding said powder between opposed compacting electroderolls, simultaneously applying pressure by means of said rolls tocompact said powder and a low voltage - high amperage currentthereacross to resistance weld said powder into a highly densecontinuous strip.
 2. The method claimed in claim 1, wherein an opensided traveling mold is interposed between said rolls to receive saidmetallic powder.
 3. The method claimed in claim 2, wherein said mold isU-shaped and one of said rolls is adapted to ride within said mold. 4.The method claimed in claim 1, including the steps of cooling said stripand winding said welded strip into coil form.
 5. The method claimed inclaim 1, wherein said rolls are preceded by at least one pair of opposedstrip compacting electrode rolls whose voltage is greater than saidfirst noted voltage, said rolls effecting a pre-welding of said metallicstrip.
 6. The method claimed in claim 5, wherein there are a pluralityof pairs of initial rolls, each pair of which is characterized by avoltage decreasing in the direction of feed of said metallic strip. 7.The method claimed in claim 6, wherein one roll from each said pair isdisposed within an encircling belt, and said belt engages the pre-weldedmetallic strip.
 8. The method claimed in claim 1, wherein saidwelding-compacting is conducted in a controlled environmentsubstantially free of oxide promoting conditions.
 9. The method claimedin claim 8, wherein said environment is substantially a dry hydrogenatmosphere.
 10. The method claimed in claim 1, including the step ofvarying the gap between said rolls, whereby to insure a uniform andfully dense strip from edge to edge.
 11. The method claimed in claim 10,wherein said variation is achieved by providing said electrode rollswith a uniform negative crown from one end to the other.
 12. The methodclaimed in claim 1, including the step of depositing said powder metalon a conveyor, whereby said conveyor is positioned to feed the powderedmetal directly between said compacting electrode rolls.
 13. The methodclaimed in claim 12, including the step of varying the depth of saidpowdered metal on said conveyor from one edge thereof to the oppositeedge.
 14. The method claimed in claim 13, wherein said variation isachieved by means of a plurality of feeder hoppers, each said hopperdepositing said powdered metal at a predetermined rate on a preselectedportion of said conveyor.
 15. The method claimed in claim 13, whereinsaid variation is achieved by interposing flat strips between thefeeding source and said conveyor.
 16. The method claimed in claim 12,including the step of subjecting said powdered metal to a compactingoperation preceding said compacting-welding.
 17. The method claimed inclaim 1, wherein said welding is accomplished in a plurality of zonestransverse to said strip, and that each said zone is individuallycontrolled so as to vary the current passing through said strip from oneedge tO the opposite edge.
 18. The method claimed in claim 2, includingthe steps of isolating said powdered metal into a plurality of narrowchannels from a location preceding up to the said compacting-welding.19. The method of producing a metallic article from powdered metalcomprising the steps of selecting a metallic powder free of non-metallicbinding agents, placing said powder between opposed compactingelectrodes, each said electrode characterized by a plurality ofindividual zones, each zone connected in electric circuit with one zoneof the opposing electrode, simultaneously applying pressure by means ofsaid electrodes to compact said powder and a low voltage - high amperagecurrent thereacross to resistance weld said powder into a highly densearticle, with said current selectively controlled in each pair ofelectrically connected zones so as to vary the current flow betweenadjacent zones in a given electrode.